Identification system for simultaneously interrogated labels

ABSTRACT

An identification system having an electronic label for processing articles such as baggage or carrier cargo. The system uses the principle of electromagnetic communication in which an interrogator containing a transmitter generates an electromagnetic field through which the electronic label containing a label receiving antenna may pass. The electronic label is attached to the article being processed and includes means for sensing the electromagnetic field and means for generating intermittently repeated label reply signals. The system includes a receiver for detecting and decoding the label reply signal. The electronic label replies intermittently as long as it is within the electromagnetic field, and the field is maintained for a period of time which is greater than the time interval between the intermittently repeated label replies. The electronic label also includes means for determining the interval between the intermittently repeated label reply signals without reference to timing signals external to the label. The interval between label reply signals varies from label to label and is greater than the time required for a label reply.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system for handling of articles suchas baggage or carrier cargo. In particular the invention relates to asystem for automated identification of articles wherein an electronicsub system called an interrogator including a transmitter and receiverextracts by electromagnetic means useful information from anelectronically coded label attached to such articles as they areprocessed through sorting operations eg. at an airport or node of acargo handling organisation.

Although the present invention is herein described with reference to abaggage/cargo sorting system it is to be appreciated that it is notthereby limited to such applications. Thus the sorting system of thepresent invention may be applied to material handling operationsgenerally eg. sorting of stock or parts.

2. Description of the Prior Art

A block diagram of the type of system to which the invention relates isshown in FIG. 1. This system uses the principle of electromagneticcommunication in which an interrogator containing a transmittergenerates an electromagnetic signal which is transmitted via aninterrogator antenna system to an electronic label containing a labelreceiving antenna. The label antenna receives a proportion of thetransmitted energy and through a rectifier generates a dc power supplyfor operation of a reply generation circuit connected either to thelabel receiving antenna or a separate label reply antenna with theresult that an information bearing electromagnetic reply signal isradiated by the label.

As a result of electromagnetic coupling between the label andinterrogator antennae, a portion of a time-varying radio frequencysignal is transmitted by the label antenna and may enter theinterrogator antenna, and in a signal separator located within theinterrogator be separated from the signal transmitted by theinterrogator, and passed to a receiver wherein it is amplified, decodedand presented via a microcontroller in digital or analog form to othersystems such as a host computer or a system of sorting gates which makeuse of the information provided by the interrogator.

In the label, operations of the reply generation circuit may becontrolled in time by an oscillator, the output of which may be usedeither directly or after reduction in frequency by a divider circuit tocontrol the code generator circuit and a reply interval generatorcircuit. The code generator circuit may control a modulator circuitwhich may present a time-varying impedance varying in accordance withthe modulation signal either directly to the receiver antenna or to therectifier, or may present a modulated reply carrier signal to a replyantenna. The code generator circuit may alternatively present to thereceiver antenna or to the rectifier a reply signal without carrierwave. The reply interval generator circuit may control timing signals tothe code generator circuit or to the modulator circuit so that the replysignal is radiated by the label for only a portion of the time for whichthe label is interrogated.

Propagation of electromagnetic signals between the interrogator antennasystem and the label antenna may be constrained to take place within afield confinement structure which may be used to enhance the coupling ofenergy between the interrogator antenna system and the label antenna,and may also be used to diminish unwanted propagation of interrogatorenergy beyond the region desired for interrogation. The interrogatorantenna system may be connected to the interrogator via an antennare-configuration switch, either mechanical or electronic, which allowsthe nature of the interrogation field created by the interrogatorantenna system at the position of the label to be changed in magnitudeand direction. Such antenna re-configuration may be automatic over timeor may be under control of the microcontroller within the interrogator.

Within the interrogator the transmitter may generate, in addition to thesignals supplied to the interrogator antenna system, reference signalssupplied to the receiver and may also generate signals supplied to afield cancellation system which may be placed externally to the fieldconfinement system or to the region occupied by the interrogator antennasystem, and may serve to reduce the net propagation of interrogationsignals beyond the region desired for interrogation. The signalssupplied to the field cancellation system may be fixed in nature or maybe varied under control of the microcontroller which may receive signalsfrom a field sensing system which samples the unwanted propagatingsignals in regions external to the interrogation region.

Within the interrogator the microcontroller may perform in addition tothe operations discussed various calculation and control functions suchas functional testing of the system components and may also participatein the reply decoding process.

In the design of practical systems for cargo and baggage handlingseveral problems can arise. One problem is that of discriminationbetween replies which can come simultaneously from identification labelsattached to different objects simultaneously present in theinterrogation field. The usual solution to this problem is to ensurethat the interrogation field is created at a very low frequency, belowthe broadcast band, by a near-field dipole antenna in which theinterrogation field strength diminishes as approximately the third powerof the distance from the antenna to the label, and to simultaneouslyensure that differently labelled objects are sufficiently separated intheir passage through the identification apparatus for the field of theinterrogator to excite only one label at a time. When such sufficientseparation is not provided, replies from different tags interfere onewith another, and incorrect readings, or a failure to read a label, canoccur.

A further problem is caused by orientation sensitivity of magnetic fieldsensitive label antennae to low frequency interrogation fields. Theproblem is compounded by unpredictability of construction of objects towhich labels are to be attached, and the modification to the fieldcreated by the interrogator which can occur in commonly occurringsituations. An example is provided by the metal clad suitcase, upon thesurface of which eddy currents are created by the interrogation field,so that the resulting oscillating magnetic field in the vicinity of thesurface is constrained to lie parallel to that surface. This fieldre-orientation, together with the fact that planar coil label antennaeare insensitive to a magnetic field within their plane, causes lowfrequency labels which lie close to and parallel to such metal surfacesnot to respond. It is also the case that simply configured magneticfield creating antennae generate field configurations with symmetryplanes through which a conveyor borne label can pass without at anystage achieving strong coupling to the interrogation field.

A further problem is that of achieving an interrogation field whichprovides an acceptably low level of interference to other users of theelectromagnetic spectrum. Commonly used solutions to this problemconsist of either the use of interrogation frequencies well below thebroadcast band, at which frequencies radiation restrictions aregenerally less stringent than at higher frequencies, and for whichradiation from magnetic field producing antennae of size useful ininterrogation of labels attached to person-portable objects is small, orthe use of UHF or microwave frequencies where in some countries bandsallowing greater stray radiation are defined.

However, each of these solutions has disadvantages in respect of bothlabel manufacturing cost and performance. When interrogation frequenciesbelow the broadcast band are used, label antennae require finely etchedpatterns of many turns to achieve the required induced voltages, andgenerally also require installation in the label of resonatingcapacitors which are of a size impractical either to apply separately atlow cost or to manufacture in monolithic integrated circuit form.Moreover the use of such low frequencies renders the weak label replieswhich also occur at low frequency difficult to distinguish against arelatively high level of electromagnetic background noise present in allurban environments.

When UHF or microwave frequencies are used, circuit components whichwill perform all the functions required to extract energy from theinterrogation signal and generate a reply become impossible both tomanufacture inexpensively and to incorporate into a single integratedcircuit manufacturing process, with the result that label manufacturingcosts again become unacceptable for mass application. At thesefrequencies there is the additional problem that reflections ofinterrogation energy and multi-path propagation can generateconcentrations of interrogation energy at regions outside the intendedinterrogation region, and ambiguities of the source of a reply canoccur. Such frequencies also suffer from both screening bywell-conducting objects and attenuation by partially conducting objectswhen the identifying label happens to be positioned with an obstructedview of the interrogation antenna.

The problems described above are compounded when the objective ofobtaining from licensing authorities type approval rather thanindividually licensed approval of automated identification and sortinginstallations, particularly in a sensitive location such as an airport,is considered. For type approval, radiation of the interrogation energybeyond the interrogation area is required to be kept at a particularlylow level, while at the same time, because the label contains no energysource so that manufacturing costs may be kept low, the interrogationfield is required to be strong enough to allow the label to derive itsoperating power therefrom. These two requirements are substantially inconflict.

Further problems arise from the weak reply which occurs in passivelabels as just described. In the presence of a weak reply, there is aneed to prevent extraneous noise from the environment from entering thereceiver where it may mask the reply. There is a need also to preventexcessive amounts of the strong interrogation signal from entering thereceiver wherein it may either do damage, or through saturation of mixeror amplifier elements, reduce sensitivity.

The usual method of keeping excessive interrogation power from enteringthe receiver is to employ a transmitter-receiver signal duplexer in theform of a directional coupler, a circulator or a bridge circuit. Suchcircuit elements only provide the necessary degree of isolation whencarefully adjusted to achieve an appropriate match between the inputimpedance of the antenna structure, and a characteristic impedance ofthe duplexer structure. Even when such isolation is achieved by carefuladjustment, introduction of objects to be sorted of significant sizewithin the field of the interrogator antenna system will change itsimpedance properties by a sufficient amount to destroy the isolationachieved. The problem is not significantly alleviated by using separatetransmitter and receiver elements because the factors just mentionedmake maintenance of a high degree of isolation between such elementsimpossible to maintain.

As well as producing a loss of sensitivity of the receiver due tosaturation, lack of sufficient isolation between transmitter andreceiver paths can produce other undesirable effects. It is frequentlythe case that in order to enhance the coupling between label andinterrogation apparatus, a single label antenna operating over arelatively narrow frequency band is used, and the reply signals occupy aportion of the electromagnetic spectrum close to the transmittedspectrum. In this situation phase noise inevitably present in thetransmitter signal will appear as background noise in the receiverchannel, and will provide an unwelcome limitation to sensitivity.

Further problems arise in the receiver structure normally used as ameans of avoiding effects of low frequency phase noise in eithertransmitter or receiver local oscillator. The receiver structurenormally used is the homodyne receiver in which the same oscillator isused as the primary generator of transmitted signal and as a localoscillator for the down-converting mixer in the receiver. Such receivershave the benefit of avoiding the effects of low frequency phase noisewhich arise when separate transmitter and receiver local oscillators areused.

However, homodyne receivers have the property that the phase between thereply signal and the local oscillator presented to the down-convertingmixer is relevant to performance, in that when those two signals are inquadrature, there is no mixer output in the desired output passband.When very low interrogation frequencies are used, the requirement tomaintain an appropriate phase relationship between the reply signal andthe receiver local oscillator can be achieved by initial design andadjustment of the interrogator circuits. As, however, interrogationfrequency rises toward the higher values desired to allow low-costprocesses to be employed in manufacture of labels, phase shifts, due tothe movement of objects to be identified within the scanning field andtime delays in the propagation path between the label and interrogatorantennas, make the phase of the reply reaching the receiver mixeruncertain, and positions of the label occur for which effectively noreply is seen. Measures need to be taken to deal with this matter.

The usual measure is to split the reply signal into two halves which aredown-converted in frequency in separate mixers, the local oscillatorsignals fed to those mixers being in phase quadrature. The outputsignals from the mixers are then either re-combined after one ismodified to suffer a further phase shift over the relevant band, or areseparately decoded, and compared in amplitude and in the decoded result,before a decision is made on whether a correct reply has been received.Each of these measures has its shortcomings, either in distortion of thesignal envelope in the in-phase and quadrature channel recombinationprocess, or in introducing additional complexity to the interrogatorthrough the requirement to provide circuits for processing of splitsignals down to and including the decoding operation.

SUMMARY OF THE INVENTION

The present invention provides an interrogation system which permitslabels of low manufacturing cost to be used in a convenientinterrogation environment providing clarity in the definition of theregion of space from which a reply is detected, good discriminationbetween labels attached to objects of all separations, and provides asufficiently low stray interrogation field to comply with type approvallicensing requirements in all countries.

The present invention may provide a label interrogation environment inthe form of a metallic wall tube, known below as an interrogationtunnel, of cross sectional dimension less than half a wave length at theinterrogation frequency, through which a non-metallic conveyor carryingobjects to be sorted may pass. The tunnel, through operating as awaveguide beyond cut-off, may provide an interrogation field confinementstructure to limit the escape into the external environment of unwantedamounts of interrogation energy, and may also provide screening of theantenna receiving the label replies from externally generatedinterfering signals. The tunnel, through its capacity substantially tolimit interrogation energy to within its boundaries, may also providefor more efficient generation of an interrogation field with limitedtotal interrogator power, and may provide interrogation field shaping toenhance discrimination between replies from labels attached to adjacentobjects submitted for identification.

In the provision of an interrogation field confinement structure asdescribed above, attenuation of the interrogation field at the pointswhere objects to be identified enter and exit the tunnel may be enhancedby insertion within the tunnel walls, at positions between theinterrogation antenna system and the ends of the tunnel, of structuresknown as chokes and having the form of quarter wave length co-axiallines, which inhibit the passage of wall currents, supporting the fieldsinterior to the tunnel, from passing along the tunnel walls in adirection of its axis.

An advantage of the use of an interrogation frequency and interrogationfield confinement structure as described above is that the region ofspace from which replies can occur is more sharply defined than whenhigher frequencies, which are subject to reflection from the environmentand multi-path propagation, are used. A further advantage of the use ofsuch frequencies and confinement structure is that both screening bywell-conducting objects and attenuation by partially conducting objectswhich occurs with higher interrogation frequencies when the identifyinglabel happens to be positioned with an obstructed view of theinterrogation antenna are avoided.

In the interrogation field confinement structure, antennae which createthe interrogation field or receive the label replies may take the formof posts in metal walled chambers attached to the tunnel andcommunicating therewith through openings in the chamber and tunnelwalls, or may take the form of rods or bars of magnetic material placedeither in such chambers or against the interior of the tunnel walls.

The present invention may provide for the creation of a labelinterrogation field by means of a current carrying loop which may beplaced adjacent to or surround the conveyor carrying objects to besorted. Such loop may be tuned by capacitors and may be loaded byresistors either to present a more convenient driving impedance or todiminish the effect of detuning or losses introduced by objects to besorted as they pass by or through the loop.

In the tuning of such loop the tuning capacitors may be placed at asingle point or may be distributed over the circumference of the loop toimprove distribution of current within the loop which would otherwise bemade non-uniform through the operation of its self-capacitance, or toreduce build up of dangerous voltages which can occur when the requiredseries reactance is placed at a single tuning point. When a distributionof series tuning capacitors around the circumference is employed,benefits of reduced radiation of electromagnetic fields to distantpoints, and reduced pickup of noise from surrounding electric fields,are also obtained.

The present invention may also employ for the creation of theinterrogation field a plurality of field creation loops carrying similarcurrents which are adjusted in magnitude and phase to generate near tothe loops an appropriate interrogation field strength, combined withdiminished radiation to far points. Such field creation structures havethe advantage that stronger interrogation fields can be created whilestill complying with stringent restrictions on generation of strayfields set by licensing authorities.

The present invention may also employ in either the screened orunscreened field creation structures described above either fixed oradaptive far-field cancellation antennae also excited from theinterrogator. The benefit of such components is that the field reductionprovided by either a tunnel structure or a system of similar-magnitudesuitably-phased loops may be insufficient to meet the more stringentlicensing regulations, particularly when the disturbance to fixed fieldconfigurations caused by the entry of objects to be sorted into theinterrogation field is considered.

The present invention may also provide interrogation field creationsystems which may either through adjustment of separate signals providedby the interrogator to the antenna elements or through operation of anantenna system re-configuration switch change the nature of theinterrogation field established by the interrogation antenna system, inrespect of the direction of the major component of the field and in theposition at which it is strongest. The advantage of such interrogationfield re-configuration is that labels which are unfavourably oriented orpositioned in respect of one field configuration and which suffer riskof not replying with sufficient strength to be read will be stronglyread by a re-configured field.

The present invention may also provide for minimisation of label readfailures by employing substantially magnetic field sensitive labelantennae, together with a label design such that labels will notnaturally lie flat against metal clad objects.

The present invention may also minimise label read failures or misreadsthough adaptive control of interrogator power. Such control may be usedto elicit replies from unfavourably oriented tags. To minimise risk ofdamage to or misreads from other labels simultaneously in aniterrogation field which has been temporarily increased in power levelto stimulate an unfavourably oriented or positioned label, the inventionmay also provide circuits within the label to ensure correct andundamaged operation of the label over a large dynamic range ofinterrogation filed strength.

The present invention may also employ for control of timing operationswithin the label an oscillator built into the microcircuit whichperforms the reply generating operations. As the interrogator frequencymay from other considerations be required to be much greater than anyfrequency required to control timing operations in reply generation, theuse of such an oscillator may avoid excessive consumption in high speedcounting circuits within the label of the small amount of power receivedby the label which can occur when timing operations are derived by thedetection and frequency division within the label of the interrogationfrequency. This provision can thus make more of the label received poweravailable for provision of the reply.

The present invention also provides for simultaneous unamibiguousdetection and decoding of replies from several labels which mayinadvertently be positioned at one time in the interrogation field. Suchsimultaneous detection is of extreme importance in security checking ofarticles such as airline baggage where it is important for human safetyto identify every baggage item loaded on an aircraft.

The present invention may also provide through control of variousaspects of label response with power level and possibly also throughcontrol of interrogation power level, measures for greaterdiscrimination between replies from closely spaced items being sorted.

The present invention may also provide for greater sensitivity inreading weak replies in the face of strong environmental noise, in theface of strong interrogation signals and in the face of disturbances tooptimum adjustment of the interrogation system which will inevitablyoccur through the introduction of objects to be sorted of anunpredictable nature and significant size into the interrogation field,and may provide for the achievement of these objectives with a varietyof reply modulation methods which may include either frequency or phaseshift keying.

The present invention may also provide for avoidance of apparently nullresponse positions in the interrogation field which can arise in somereceiver systems as a result of phase shifts in the interrogation andreply signal propagation paths.

The present invention may also provide for maintenance of systemperformance and economy of interrogator power when objects which cansignificantly detune the interrogation antenna enter the scanned region.

The present invention may also provide for convenient and secureprogramming or re-programming of all or part of label information eitherat the time a label is first brought into use or dynamically during itssubsequent deployment in sorting operations.

The present invention may also provide for assured separate detection ofand correct reading of all replies from labels which are simultaneouslypresent in the interrogation field, even when such labels have virtuallyidentical electronic performance and are programmed with identicalinformation content.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the accompanying drawings wherein:

FIG. 1 shows major sub-units of an electronic label identificationsystem in a baggage or cargo sorting application;

FIG. 2 shows a block diagram of one form of coded label;

FIGS. 3A-3B show various forms of antenna suitable for use in low costcoded labels;

FIGS. 4A-4B show further detail of circuit functions provided in oneform of a coded label;

FIG. 5 shows further detail of circuit functions provided in anotherform of a coded label;

FIG. 6 shows further detail of circuit functions provided in yet anotherform of a coded label;

FIG. 7 shows further detail of circuit functions provided in a stillfurther form of a coded label;

FIG. 8 shows an arrangement of components in an interrogation systememploying a waveguide beyond cut-off as a field confinement structureand employing externally placed stray field cancellation antennae;

FIGS. 9A-9D show a field confinement structure employing chokes in thetunnel walls and also shows an arrangement of ferrite interrogationantennae;

FIG. 10 shows an arrangement of interrogator antenna coils tuned tominimise build up of voltage and collection of extraneous noise, andalso connected to reduce radiation to the far field, and also shows aninterrogator system providing stray field cancellation;

FIG. 11 shows a block diagram of an interrogator providing adaptiveadjustment of interrogator power level in response to perceived labelreplies, and also providing stray field sensing and cancellation.

FIG. 12 shows a block diagram of another form of interrogator;

FIGS. 13A-13D show construction of an inexpensive label which will notlie flat against metal surfaces; and

FIGS. 14A-14B show the construction and response of a receiver filterproviding for suppression of strong interrogation signals, forelimination in homodyne receivers of positional nulls in the detectionof label replies, and for minimisation of the effects of external noise.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an arrangement of an interrogator system in which aninterrogator 1 containing a transmitter 2 generates an electromagneticsignal 3 which is transmitted via an interrogator antenna system 4 to anelectronic label 5 containing a label receiving antenna 6. The labelantenna 5 receives a proportion of the transmitted energy and throughoperation of a rectifier 7 generates a dc power supply for operation ofa reply generation circuit 8 connected either to the label receivingantenna 6 or a separate label reply antenna 9 with the result that thean information bearing electromagnetic reply signal 10 is radiated bythe label 5.

As a result of electromagnetic coupling between the label andinterrogator antennae, a portion of a time-varying radio frequencysignal transmitted by the label antenna 9 may enter the interrogatorantenna 4 and in a signal separator 11 located within the interrogator 1be separated from the signal transmitted by the interrogator 1 andpassed to a receiver 12 wherein it is amplified, decoded and presentedvia a microcontroller 13 in digital or analog form to other systems suchas a host computer or a system of sorting gates which make use of theinformation provided by the interrogator.

In the label 5, operations of the reply generation circuit 8 may becontrolled in time by an oscillator 14, the output of which may be usedeither directly or after reduction in frequency by a divider circuit 15to control a code generator circuit 16 and a reply interval generatorcircuit 17. The code generator circuit 16 may control a modulatorcircuit 18 which may present a time-varying impedance varying inaccordance with the modulation signal, either directly to the receivingantenna 6 or to the rectifier 7 or may apply a modulated reply carriervoltage to reply antenna 9. The code generator circuit may alternativelypresent an impedance directly varing in accord with the reply signal,either to the receiving antenna 6 or to the rectifier 7, or may presenta reply signal directly to a separate reply antenna 9. The replyinterval generator circuit 17 may control timing signals to the codegenerator circuit 16 or to the modulator circuit 18 so that the replysignal is radiated by the label 5 for only a portion of the time forwhich the label 5 is interrogated.

Propagation of electromagnetic signals between the interrogator antennasystem 4 and the label antenna 6 may be constrained to take place withina field confinement structure 19 which may be used to enhance couplingof energy between the interrogator antenna system 4 and the labelantenna 6, and may also be used to diminish unwanted propagation ofinterrogator energy beyond the region desired for interrogation. Theinterrogator antenna system 4 may be connected to the interrogator 1 viaan antenna re-configuration switch 20, either mechanical or electronic,which allows the nature of the interrogation field created by theinterrogator antenna system 4 at the position of the label 5 to bechanged in magnitude and direction. Such antenna re-configuration may beautomatic over time or may be under control of the microcontroller 13within the interrogator 1.

Within the interrogator 1 the transmitter 2 may generate, in addition tothe signals supplied to the interrogator antenna system 4, referencesignals supplied to the receiver 12 and may also generate signalssupplied to a field cancellation system 21 which may be placedexternally to the field confinement system 19 or externally to theregion occupied by the interrogator antenna system 4, and may serve toreduce net propagation of interrogation signals beyond the regiondesired for interrogation. The signals supplied to the fieldcancellation system 21 may be fixed in nature or may be varied undercontrol of the microcontroller 13 which may receive signals from a fieldsensing system 22 which samples unwanted propagating signals in regionsexternal to the interrogation region.

Within the interrogator 1 the microcontroller 13 may perform in additionto those discussed various calculation and control functions such asfunctional testing of system components and may also participate in thereply decoding process.

FIG. 2 shows one preferred embodiment of a coded label. In the preferredembodiment shown in FIG. 2 a label body 23 is constructed from alaminate of thin cardboard and plastics film. The coded label includes alabel antenna 24 comprising an aluminium foil pattern embossed on to theplastics film, as does printed matching element 25. Depending upon thestyle of antenna element and matching element used, one or both sides ofthe laminate may carry aluminium conductors, and through-connections maybe made by stamped holes filled with conductive adhesive. Depending uponantenna style, operating frequency, and space available within theintegrated microcircuit 26, the printed matching element may be absentand is desirably so. Neither the antenna 24 or the matching element 25require conductors of fine resolution nor positioning.

The antenna 24 and matching element 25 are connected to the integratedmicrocircuit 26 via a pair of terminals, conductive adhesive being againthe preferred mode of connection. Within the integrated microcircuit 26an integrated matching element 27, preferably a capacitor, is connectedin parallel with the antenna 24 and printed matching element 25terminals. The system of antenna 24, printed matching element 25 andintegrated matching element 27 may form a resonant circuit at theinterrogation frequency so that coupling between the interrogator andthe label is enhanced.

The signal developed across the just mentioned resonant circuit isconveyed within the microcircuit 26 to an integrated rectifier 28 inwhich both rectifier diode and reservoir capacitor are manufactured aspart of the microcircuit fabrication process. The voltage developedacross a resevoir capacitor of the rectifier system is fed to a replygeneration circuit 29 the output of which is conveyed either directly tothe antenna system or is presented to the rectifier system.

The operation of the rectifier circuit may be such as to draw from thelabel reply antenna non-sinusoidal currents, which may result in theemission from the label of signals at one or more of the harmonics ofthe interrogation frequency, these harmonic signals being detected bythe interrogator.

Preferred embodiments of label antennae and printed matching elementsare shown in FIGS. 3A and 3B. In one preferred embodiment shown in FIG.3A the label antenna is in the form of a electric dipole 30 againfabricated as stamped aluminium foil. The printed matching element takesthe form of a rectangular spiral tuning inductor 31 connected inparallel with the antenna 30. The parallel combination is connected tothe integrated microcircuit 26 via a pair of stamped aluminium foilconductors.

In another preferred embodiment shown in FIG. 3B the antenna consists ofa magnetic dipole in the form of a large area spiral pattern loop 32with only a small number of turns. The printed matching element takesthe form of a tuning capacitor 33 fabricated as a pair of aluminium foilplates with the plastic film of the label body 23 as the dielectric.Necessary through-connections and connections to the microcircuit 26 arepreferably made by stamped holes and conductive adhesive. At thepreferred interrogator operating frequency of 27 MHz the rectangularportion of the label antenna can be 50 by 80 mm with the magnetic dipolecontaining four turns each of strip width 0.8 mm and spacing 0.8 mm, andthe printed tuning capacitor is absent.

The ability to use low resolution printed antenna components andexternal matching elements, together with only a two-wire connectionbetween the microcircuit, which may contain additional matchingelements, and the antenna structure is in part a consequence of thechoice of an operating frequency in the region of 3 to 30 MHz and hasthe advantage that label manufacturing costs are kept low.

More detail of the construction of the preferred embodiment of the labelshown in FIG. 3B, and in particular of the functions performed withinthe integrated microcircuit, is shown in FIG. 4A.

In this preferred embodiment the reactance of the magnetic dipoleantenna 32 is removed by an integrated matching element 33 consisting ofa capacitor, preferably formed either with silicon dioxide dielectricbetween polysilicon layers or using gate oxide dielectric between apolysilicon layer and an implanted region of the substrate. Theoscillating voltage developed across the antenna system is converted toa dc supply for the microcircuit 26 through the integrated rectifierdiode 34 and integrated reservoir capacitor 35.

The output of rectifier 34 is presented to an unmodulated variable load36. The unmodulated variable load 36 operates under control of a voltagesensor 37 so that as the voltage developed by the rectifier 34 risesabove the value for optimum circuit operation, a decreasing impedance ispresented by the unmodulated variable load 36 to the rectifier 34. Theresult of the additional loading will reduce the quality factor of theresonant circuit formed by the magnetic dipole antenna and its matchingelement with the result that the label will be protected againstincorrect functioning or damage from strong interrogation fields whichare encountered by labels which pass with optimum orientation close tothe interrogator antenna.

The advantage of providing this feature is that interrogator fields ofgreater strength may be used, either fixed in time or adaptively varied,to elicit replies from badly positioned or orientated labels.

The voltage developed by the rectifier 34 is also presented to andpowers an integrated circuit oscillator 38. In the preferred embodimentdescribed here this oscillator will operate in the frequency range 180kHz to 220 kHz, and will provide among other things a reply signalsub-carrier signal.

An advantage of using within the label an integrated oscillator tocontrol timing operations is that the high power consumption associatedwith circuits which derive their timing from counting down therelatively high interrogation frequency is avoided, and more of thepower present in the label is available for generating the reply or forcontrolling essential processes which generate the reply by backscatterof a portion of the energy received by the label antenna.

The output of integrated circuit oscillator 38 is fed to a bit ratedivider 39 which in the preferred embodiment shown here will accomplishfrequency division by a factor of 4. The output of the bit rate divider39 is fed to a message interval circuit 40 which controls the operationof and receives information from a reply code generator circuit 41containing a sixty-four bit reply code stored in a reply code memory.The information in the reply memory may be fixed in content at the timeof manufacture, may be programmable at the time the label is broughtinto use, or may be re-programmed in operation, and may also contain aunique microcircuit serial number which may be made part of the reply ormay be used for other purposes.

The information in the reply code memory may be fed, in part, at the bitrate, to a modulator circuit 42 in which the reply information ismodulated upon a reply sub-carrier provided by the integrated circuitoscillator 38.

The relationship between the message interval circuit 40 and the replycode generator circuit 41 is in part as follows. When the circuit is inunpowered state and receives power from an interrogation signal, themessage interval circuit 40 asserts a Reply Code Generate signal whichis passed to the reply code generator 41 which then issues, at the replybit rate, four repetitions of the reply information stored in the replycode memory to the modulator, and also issues appropriate information tobe defined below to the message interval circuit. Other aspects of theinteraction between the message interval circuit 40 and the reply codegenerator circuit 41 are discussed further below.

In the preferred embodiment being described here the output of the replycode generator 41 is combined with the output of the integrated circuitoscillator 38 in the modulator 42 to accomplish differential phase shiftkeying of the reply sub-carrier frequency, a phase change in the outputof the modulator occurring at the end of each four cycles of the replysub-carrier whenever the output of the reply code generator 41 is abinary zero.

The output of the modulator 42 together with the output of the voltagesensor 37 is applied to modulated variable load 43. This componentpresents to the power supply line a time-varying impedance which variesin accord with a signal from the phase modulator 42 and also varies within amplitude with the signal derived from the voltage sensor 37. Theresult is that a time-varying loading is presented first to therectifier 34 and is then presented by the rectifier 34 to the labelantenna 32. It will there cause generation, through the mechanism ofmodulated backscatter described below, of a reply signal from the label.

In this embodiment the reply signal may consist of modulated sidebandsof the interrogator signal or modulated sidebands of a harmonic of theinterrogator signal.

The variable impedance presented to the label antenna will provide thata portion of the available power received by the antenna will not betransmitted to its rectifier load, but will be reflected back into theantenna, wherefrom it will be re-radiated in a time varying manner. Thistime varying re-radiated signal will consist of sidebands of theoriginal interrogation frequency, separated therefrom by a frequencydifference equal to the frequency of the integrated circuit oscillator.The sub-carrier signal represented by those sidebands will be phasemodulated with the reply signal information at the rate of one bit perfour cycles of sub-carrier frequency.

In the reply generation process described above, the amplitude of theimpedance variation provided by the modulated variable load 43 is madedependent upon the output of the voltage sensor 37 so that, when anunmodulated variable load 43 presents a low impedance to rectifier 34,the relative level of unmodulated and modulated impedance may beapproximately preserved, and the relative strength of the backscatteredreply will not diminish.

The advantage of the method of reply code generation discussed above isthat the label may be operated without damage or malfunction over a widedynamic range of interrogation signal.

At the conclusion of the reply code generation period, the Reply CodeGenerate signal is de-asserted and the generation of a reply signal bythe label temporarily ceases. However, during the reply code generateperiod the message interval circuit receives data, either the reply codeor a microcircuit serial number, from the reply code generator, and usesthat data to initialize the state of a random number generator which isused to determine the number of reply bit periods which elapse beforethe Reply Code Generate signal is re-asserted and the reply signal isre-radiated by the label.

In the preferred embodiment being described here, the interval betweenreply periods is designed to be pseudo-randomly distributed between zeroand 40 ms. That interval provides on the average an adequate time forreplies from other labels simultaneously in the interrogation field tooccur without interfering with the reply from the label underdiscussion. Even when such interference between replies does on oneoccasion occur, the differently seeded pseudo-random distribution ofreply intervals within different labels will ensure that the probabilityof this interference continuing to occur will with time becomevanishingly small.

An alternative preferred embodiment of the label is provided in FIG. 4B.In this embodiment the reactance of the magnetic dipole antenna 32 isremoved by the integrated tuning capacitor 33 placed within theintegrated microcircuit 26. From the voltage developed across theantenna 32 and tuning capacitor 33 a power supply for the operation ofthe microcircuit is developed by the integrated rectifier diode 34 andintegrated reservoir capacitor 35. Again an unmodulated variable load 36operating under a control of voltage sensor 37 serves to keep thedeveloped power supply within the microcircuit within acceptable levelsin the face of large variations in strength of interrogator field. Thepower supply is again presented to a integrated circuit oscillator 38which in this embodiment has an enhanced sensitivity of frequency tosupply voltage and operates preferably in the range of 150 kHz to 250kHz.

The output of the integrated circuit oscillator 38 is passed to a bitrate divider 39 and a message interval circuit 40. In this embodimentthe message interval circuit 40 simply consists of a dual counter whichalternately issues a high signal for 256 of the reply bit periods, and alow signal for 4096 reply bit periods.

The outputs of the bit rate divider 39 and the reply interval circuit 40are both passed to a reply code generator 41 which performs functionssimilar to those described for FIG. 4A, except that the information fromthe reply code generator 41 is not in this case conveyed to the replyinterval circuit 40. In this embodiment the output of the reply codegenerator 41 is presented to the integrated circuit oscillator 38wherein it accomplishes, at the bit rate, changes to the oscillatorfrequency in addition to the relatively slow changes accomplished byvariations in supply voltage.

The result is that the frequency of integrated circuit oscillator 38 isslowly varying with the microcircuit supply voltage and is in additionfrequency modulated by the contents of a reply code memory within thereply code generator 41. The output of the integrated circuit oscillator38 is presented, together with the output of the voltage sensor 37, tothe modulated variable load 43 the operation of which is to generatethrough the mechanism of modulated backscatter, a reply code in themanner described in relation to FIG. 4A.

In the preferred embodiment described in relation to FIG. 4B the voltagedependence of the integrated circuit oscillator within the microcircuitoperates, together with the fact that the reply message isintermittently issued at intervals which vary inversely with thefrequency of that oscillator, to ensure that whenever several labels arepresent within interrogation field, the natural variation between labelcircuits, together with variation in strength of interrogation fieldfrom one position to another within the interrogation environment,replies from different tags within the field will not permanentlyoverlap.

This feature of variation in the reply intervals between differentlabels may be enhanced by varying the power level of the interrogationfield, as it is a property of most integrated circuit oscillators thatthe variation of oscillation frequency with power supply voltageincreases as the supply voltage is reduced towards the threshold ofoscillation.

By the deployment of the range of measures described the probability offailure to read all labels passing through the scanned environment maybe reduced to a negligible value.

The advantage of this provision of a reply code interval in the waysdiscussed above together with other aspects of the invention such as theprovision of enhanced dynamic range and adaptive control of interrogatorpower is that a high degree of probability may be attached to theability of the system to read all labels passing through theinterrogation field, even when labels are programmed to provide the samereply.

The basic structure of yet another preferred embodiment of the label isshown in FIG. 5. In this embodiment the receiving antenna 44 whichreceived energy from the interrogation signal has its reactance removedby the combination of printed receiver matching element 45 andintegrated receiver matching element 46.

The voltage developed across that combination of elements is convertedto a dc supply by an integrated rectifier 28 within the integratedmicrocircuit 26 and supplies power to the integrated reply generationcircuit 29. The integrated reply generation circuit 29 develops a phasemodulated reply signal at a frequency unrelated to that of theinterrogation signal. The reply signal is presented to the reply antenna47, which has its reactance removed by the integrated reply matchingelement 48 and printed reply matching element 49.

In the preferred embodiment described here which is designed to operatewith an interrogation frequency of 27 MHz the printed receiver matchingelement 45 may be absent. In the preferred embodiment described here thereply frequency is preferably one for which low power integratedoscillator circuits and low cost printed antenna elements are bothpracticable. In this preferred embodiment a frequency in the vicinity of2 MHz is suitable.

Operations within the microcircuit of the embodiment shown in FIG. 5 areillustrated in FIG. 6. In this embodiment the receiving magnetic dipoleantenna 50 again consists of a rectangular coil of four turns of stampedaluminium foil of strictly 0.8 mm in separation 0.8 mm. The label isinterrogated at a preferred interrogation frequency of 27 MHz. Therequired integrated receiver tuning capacitor 51 contained within theintegrated microcircuit 26 is preferably realised with polysiliconelectrodes separated by silicon dioxide dielectric. The integratedrectifier diode 34 is formed between a p type implanted layer and an ntype substrate with the reservoir capacitor again employing eitherpolysilicon electrodes and silicon dioxide dielectric or gate oxidedielectric with one polysilicon electrode and an implanted region of thesubstrate as another. An unmodulated variable load 36, operating undervoltage sensor 37, performs functions corresponding to those describedin relation to the preferred embodiments of FIGS. 4A and 4B.

In this embodiment the integrated circuit oscillator 38 will operate ata frequency of approximately 2 MHz and will be reduced in frequency in abit rate divider 39 by a factor of 32 to produce a bit period ofnominally 16 microseconds. The output of the bit rate divider is passedto the message interval circuit 40, which operates as describedpreviously in relation to FIG. 4A to provide an interval between replieseffectively randomly distributed between zero and 40 ms.

During the active period the output of the reply code generator 41varies between low and high states and accomplishes frequency modulationof the integrated circuit oscillator 38. During the inactive period theoutput of the integrated circuit oscillator 38 remains undisturbed,other than to follow its normal variations with respect to power supply.

In this embodiment the frequency modulated output of the integratedcircuit oscillator 38 is presented directly to the transmitting magneticdipole antenna 52 via additional output connections one of which isseparate from those to the receiving magnetic dipole antenna 50. Thereactance of the transmitting magnetic dipole 52 is removed by acombination of printed matching element 53 taking the form of analuminium foil plastic dielectric capacitor and integrated tuningcapacitor 54.

Operations within the microcircuit of another preferred embodiment ofthe label shown in FIG. 2 are shown in FIG. 7. In this embodiment whenpower is first applied by the interrogator to the circuit in aninitially unpowered state, the sub-units 55, 57, 58, 63 and 64 areinactive and sub-units 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 43function as described previously in relation to FIG. 4B to generate areply.

When in this condition, however, the interrogator power is abruptlyremoved so that it falls in a period of five microseconds to a value ofless than half the value established in providing the reply, and then ina period of ten microseconds is re-established, the voltage falldetector 55 detects this condition and sends an Enter Program Modesignal 56 to program charge pump 57 and program control circuit 58.

Upon the receipt of this signal the program charge pump 57 becomesactive and begins to generate through the mechanism of charge pumping aprogramming voltage higher than the normal circuit operating voltageprovided by the rectifier diode 34 and reservoir capacitor 35, andsupplies that voltage to the reply code generator 41 which in thisembodiment contains at least part of the reply code in an electricallyre-programmable memory. At the same time the program control circuit 58sends a signal to the reply code generator 41 to prevent until theprogramming process is complete the generation of the reply code.

For further operations in the microcircuit the interrogation signal mustbe modulated in amplitude with a depth of at least 20% and at a bit ratecorresponding to the bit rate of the reply previously issued by thelabel. During this operation the interrogator power level ispredictively adjusted to bring that bit rate to the nominal 50 kbits persecond for this design of label.

The form of modulation of the interrogator power is pulse widthmodulation with a binary one being signalled by a high power level for aperiod of 15 microseconds followed by a low power level (less than 80%of full power) for a period of 5 microseconds, while a binary zero issignalled by a low power level (less than 80% of full power) for aperiod of 15 microseconds followed by a high power level for a period of5 microseconds.

With the aid of the integrated circuit oscillator 38 to provide a timingreference, this form of modulation is detected by the voltage falldetector 55, and is passed to the program control circuit 58. Thatcircuit records the first 64 binary digits signalled to the label andinterprets them as a programming mode key. In this embodiment theinterpretation of those digits employs the principle of feedback shiftregister encoding using the reply code as a key, so that for each labelthe correct value of the key depends upon its programmed contents and iscalculated in the interrogator from the just received reply before it isissued. The advantage of this method of keying is that the relationbetween the code and the key may be kept confidential between the labelmanufacturer and accredited users, and greater security in theprogramming operation results.

Following the receipt of a correct programming mode key, the labelinterprets the next portion of the programming bit stream as data to beprogrammed into reply memory. Providing the conditions relating toProgram Lock and Program Enable bits described below are satisfied, thatdata is then with the aid of Programming in Progress Signal 59, WriteData Signal 60, Write Enable Signal 61 entered into the reply codememory of reply code generator 41, taking a time appropriate to theprogramming speed of the memory. Upon the completion of the entry of theprogrammed data the reply code generation functions of the label arerestored, and the interrogator checks that correct programming has takenplace.

The operations described above will only occur when two bits of thereply code memory, known as the Program Enable Bit and the Program LockBit, are in the appropriate states. The Program Enable Bit, which canwhen the Auxiliary Programming Signal to be described below is applied,be both raised and lowered by the programming process described above,must be in a high state, while the Programming Lock Bit, which when theAuxiliary Programming Signal described below is applied can be raised bythe programming process described above but once raised cannot belowered by any means, must be in a low state.

For the Auxiliary Programming Signal to be asserted on the label asecond electromagnetically coupled signal must be applied to the labelantenna. This auxiliary signal is applied via a carrier at a frequencyof approximately 500 MHz, and is applied to the label in a programmingchamber which may take the form of a closed metal container or awaveguide beyond cut-off. The second electromagnetically coupled signalenters the label via the same terminal pair as the interrogation signal.Within the label use is made of the high ratio of the frequency of thesecond signal to the normal interrogation frequency for it to beseparated in a multistage resistance-capacitance high-pass filter 62,whereupon it is rectified in rectifier 63 to generate the ProgrammingPermit Signal 64 which is relayed to the programming control circuit 58to accomplish the operations aforesaid.

The advantage in programming in the way described above are that: with asingle and economical design of label, programming operations can bemade highly secure; programming operations can include all variationsfrom one-time programming to frequent re-programming of a label while inuse; and as only a single terminal pair is used for connection of thelabel circuit to the antenna structure on the label body, labelmanufacturing costs are kept at a minimum.

FIG. 8 shows a preferred embodiment of an object identification systemcontaining a field confinement structure and a field cancellationstructure. In this embodiment, baggage items 65 with attached electroniclabels 5 are placed on non-metallic conveyor 66 through a rectangularmetallic tunnel 67 which serves as the field confinement structure. Inthis embodiment the tunnel 67 has an interior width of 900 millimetres,an interior height of 1 metre, and a length of 2 metres, andinterrogation takes place at a frequency of 27 MHz, at which frequencythe electromagnetic wavelength is much greater than both aperturedimensions of the tunnel 67.

In this embodiment the interrogation field is created by rectangular barantennae approximately 100 millimetres in square cross section, attachedto each side wall and with a 20 millimetre gap at the centre providing afeed point for that antenna. From the feed point a coaxial line passesthrough the interior of one bar to the outside wall of the structure,thus permitting shielded connection of that antenna to theiterrogator 1. In this embodiment the fields created by the antennae aresubstantially magnetic and follow a path surrounding the antenna bars inclosed loops. A portion of the field extends into the interior sectionof the tunnel 67 with the return flux path being provided by themetallic wall antenna housings 68 which provide a vertical expansion ofthe tunnel 67 cross section in the vicinity of its centre.

As well as being connected to the interrogation field creation antennae,the interrogator 1 has connections also to field cancellation antennae21 placed close to each end of the tunnel mouth. The field cancellationantennae 21 are capable of radiating both vertically and horizontallypolarized electromagnetic fields, the radiation being adjusted to cancelin the far field region of the residual radiation emerging from each endof the tunnel mouth.

The advantages of the provision of an interrogation field, fieldconfinement structure and field cancellation system in the mannerdescribed above are manifold. They include the enabling of labelmanufacture at very low cost; the achievement of particularly low strayinterrogation fields so that type approval of systems may be obtained;economy of power in the creation of interrogation fields of appropriatestrength; the avoidance of multi-path ambiguities in the position fromwhich replies have been obtained; elimination of reading failures fromlabels with an obstructed view of the interrogator antenna; andeffectively complete elimination of effects of environmental noise.

In one preferred embodiment of the invention the interrogator 1 shown inFIG. 8 may contain a number of independent transmitter, receiver andreply decoder systems, connected separately to interrogator antennas 68,the outputs of the said decoder systems being combined in one of thedecoders or in an overall system controller within the interrogator sothat for each correctly decoded label a single report of its decodedreply is provided to the sorting gates or host computer.

In an alternative embodiment of the field creation structure the barsare increased in width to 300 mm and are driven in phase, or inquadrature. The adjustment of the configuration and phasing of theantennas, the overlap of the antenna fields, and the angular nature ofthe field enclosure where the antenna housings meet the tunnel sidewalls, all serve to create regions of rapid spatial variation in theamplitude and direction of the antenna field. This shaping of theantenna field has the advantage that replies from tags in particularorientations are confined to short intervals of time as a label passesthrough the antenna region with the result that the order of entry oflabels into the interrogation region may be inferred with increasedaccuracy compared with field generation systems creating spatially moreuniform fields.

An alternative preferred embodiment of the field confinement structureand interrogation field antenna structure is illustrated in FIGS. 9A-9D.FIG. 9A shows a sectional view on BB of FIG. 9B and FIG. 9B shows asectional view on AA of FIG. 9A. FIG. 9C shows a sectional view on CC ofFIG. 9A and FIG. 9D shows a sectional view on DD of FIG. 9A. In thisembodiment a waveguide beyond cut-off of dimensions similar to thatdiscussed in FIG. 8 for an operating frequency of approximately 27 MHzis again used. However, in this embodiment the interrogation fieldcreation antennae take the form of ferrite bars 69 excited by ferriteantenna windings 70 and placed in shallow chambers 71 exterior to thetunnel walls and of length a little less than the height of the tunnel,and communicating therewith through rectangular apertures 72 whichprovide an entry for the magnetic flux within the ferrite bars 69generated by the antenna excitation system.

The confinement of the interrogation fields substantially to the regionof the tunnel is achieved in part by again exploiting the properties ofelectromagnetic field distributions in waveguides beyond cut-off and inpart by the insertion of current inhibiting chokes 73 in series with thetunnel walls between the region occupied by the interrogation antennastructure and the tunnel openings. The chokes illustrated take the formof a re-entrant coaxial lines, short circuited at one end and of totallength one quarter of a wave length so that at the aperture 74 at whichthey connect to the interrupted tunnel walls an open circuit ispresented.

These chokes 73 serve to inhibit the passage of wall currents across thegap and thus aid in the confinement of the interrogation field to thecentral region of the tunnel.

The advantages of the provision of an interrogation field and fieldconfinement structure in this way are: that reduction of stray fieldsand reduction of environmental noise, even more than is provided by thepreviously discussed tunnel can be achieved; that more compact antennaecan be realised; and that in consequence additional differently orientedantennae can be introduced, such antennae being operated in phase, inanti-phase or in quadrature to achieve minimization of weak fieldpositions and unfavourable label orientations, all without significantpenalty in the form of space occupied by the structure.

A preferred embodiment of yet another type of interrogation fieldcreation and cancellation structure is illustrated in FIG. 10. In thisembodiment baggage items 65 carrying electronic labels 5 enter on aconveyer 66 which passes through a pair of single-turn portalinterrogation antennae 75 in which reactance is removed by a number ofseries tuning capacitors 76 disposed at intervals around thecircumference of the loops. The loop antennae are spaced in thedirection of the conveyer by distance approximately equal to the widthand are excited by the interrogator 1 so that currents 77 flow inopposite directions in the two loops. On top of the interrogator smallerbut similarly oriented field cancellation antennae 78 are mounted.

In this embodiment the field created by the interrogation antenna systemis substantially four pole in nature with a maximum logitudinalcomponent in the center of each loop and a null field half way betweenthem. In this embodiment the disposition of currents and the positionsof the antennae are such as to minimize radiation to the far field. Suchradiation as does occur is substantially cancelled by suitably phasedadditional currents which flow in the field cancellation antennae 78mounted on the interrogator 1 or elsewhere if desired. The fieldcancellation antennae are sufficiently small for their own near fieldsnot to significantly reduce the interrogation field at the labelposition.

The advantages of this form of field creation and stray fieldcancellation are: economy of space occupied by and economy in cost ofmanufacture of an interrogation installation; reduction in radiation ofinterrogation energy to far fields as a result firstly of the use ofdistributed series tuning capacitors, secondly as a result of themagnetic four pole nature of the field creation structure, and thirdlyas a result of the stray field cancellation system, so that typeapproval of an interrogation installation may be more easily obtained;as a result of the distributed capacitance of the tuning system areduction of both dangerous voltages and unwanted electric fields in theinterrogation region; and as a consequence of the interrogation fielddistribution the obtaining of two interrogation opportunities for onepass of a label through the interrogation antenna structure. The fieldshaping which results from overlap of the antenna fields and therelative amplitude and phasing of the antenna currents to produce afield null between the loops offers the advantage of more accurateinference of relative label position as labels pass through theinterrogation region.

A block diagram of a preferred embodiment of an interrogator system isprovided in FIG. 11. In this system the interrogation signal isgenerated by a local oscillator 79, increased in amplitude in atransmitter signal amplifier 80 and passed through signal separator 11to interrogator antenna system 4.

The reply signal from the labels is received by the same interrogatorantenna system and separated by signal separator 11 and passed viareceiver filter 81 to balanced modulator 82. In the preferred embodimentthe receiver filter provides very large attenuation, of the order of 60dB, at the interrogation frequency while providing a passband 100 kHzwide situated 200 kHz above the interrogation frequency for the replysignal.

The advantages of a receiver filter with these properties are that:reduction in receiver sensitivity through saturation of thedown-converting mixer is avoided; the introduction of interrogatorsignal phase noise into the receiver path is reduced; the influence ofenvironmental noise in the receiver is reduced; and positional nulls inthe label reply which result when double sideband homodyne reception isused are eliminated.

The balanced modulator 82 also receives a signal from the localoscillator 79 to effect down-conversion of the reply signal to a moreconvenient amplifying frequency. The output signal of balanced modulator82 is passed to an amplitude detector system 83, the output of whichenters microcontroller 13 to serve as an indication of whether replieshave been obtained or some adjustment to interrogator power is required.

This adaptive adjustment of interrogator power has the advantage ofeliciting replies from unfavourably oriented or positioned labels.

The output of balanced modulator 82 is also passed to a low frequencyband pass amplitude limiting amplifier 84 which provides amplificationin the region of the reply sub-carrier, and also provides suppression ofsmall signals.

The advantage of using an amplitude limiting amplifier, and frequency orphase modulation in the encoding of the label reply, are that when morethan one label has responded only the strongest supply is significantlypresent at its output, and that the preferably phase or frequency ormodulated reply signals are presented at a suitable amplitude to signalsampler 85.

The signal sampler 85 takes limited resolution samples at a frequencysignificantly greater than the reply sub-carrier frequency so that adigital representation of the reply signal is passed to microcontroller13. This method of operation offers advantages in that before thefrequency or phase encoded reply information is extracted, decodingalgorithms within microcontroller 13 can detect and compensate for timewarping of the reply signal sub-carrier frequency which can occur aslabels move through the interrogation field which varies in strengthfrom position to position.

The microcontroller 13 also controls the gain of a cancellation signalamplifier 86 of which the input signal is the derived from the localoscillator 79 and the output signal is conveyed to field cancellationantennae 87 to give the advantage of minimisation of stray radiationwhich escapes the field confinement system. The level of thecancellation field is determined by the microcontroller 13 whichreceives information from stray field sensing antennae 21 via strayfield sensing amplifiers 88 and stray field sensing detectors 89.

The microcontroller 13 may also control an antenna re-configurationswitch which can place the excitations of antennae on opposite sides ofthe tunnel in phase, in anti-phase or in quadrature to provide theadvantage of minimization of weak field positions and unfavourable labelorientations within the structure.

In an alternative preferred embodiment the receiver filter 81 providesits passband 200 kHz above or below the second harmonic of theinterrogation frequency, and the balanced modulator 82 is fed with alocal oscillator signal at the second harmonic of the interrogationsignal.

An advantage of providing for reply detection in the band surrounding anharmonic of the interrogation signal is that harmonic energy and phasenoise of the transmitter signal, which in homodyne receivers present alimit to system sensitivity, may in this elevated reply passband beeasily removed from the transmitter signal by low cost filter circuits.

A further advantage of the use of a reply passband near an harmonic ofthe interrogation frequency is that local scattering systems such aselectrically powered machinery which can interact readily with theinterrogation signal to produce at sidebands of the interrogationfrequency intefering signals which can compete with the label reply, donot as readily produce interfering signals at sidebands an harmonic ofthe interrogation frequency, and label replies can be detected in thepresence of strong local scatterers.

A block diagram of another preferred embodiment of an interrogatorsystem is provided in FIG. 12. In this system the interrogation signalis generated by a transmitter master oscillator 90, increased inamplitude in a transmitter signal amplifier 80 and passed throughfrequency selective signal separator 91 to interrogator antenna system4. The reply signal from the labels is received by the same interrogatorantenna system and separated by signal separator 91 and passed viareceiver filter 81 to balanced modulator 82. While no longer requiring asharp notch at the interrogation frequency in its response, thisreceiver filter 81 has the task of sharply rejecting signals at theinterrogation frequency while providing an appropriate passband for thereply signal.

The balanced modulator 82 also receives a signal from a receiver localoscillator 92 to effect down-conversion of the reply signal to a moreconvenient amplifying frequency.

The output signal to the balanced modulator 82 is passed to an amplitudedetector system 83 the output of which enters the microcontroller 13 toserve as an indication of whether replies have been obtained or someadjustment to the interrogator power is required. The output of thebalanced modulator 82 is also passed to a low frequency band passamplitude limiting amplifier 84 which provides amplification in theregion of the reply frequency base band and also suppression of smallsignals to ensure that when more than one label has responded only thestrongest supply is significantly present at its output, and also topresent the preferably frequency modulated reply signals at a suitableamplitude to the signal sampler 85. This component takes limitedresolution samples at a frequency significantly greater than the replysub-carrier frequency so that a digital representation of the replysignal is passed to the microcontroller 13. This method of operationoffers the same advantages as were discussed in relation to FIG. 11.

The microcontroller 13 also controls the gain of a cancellation signalamplifier 86, the input signal of which is the derived from masteroscillator 90 and the output signal is conveyed to field cancellationantennae 87 whose purpose is to cancel stray radiation which escapes thefield confinement system. The level of the cancellation field isdetermined by the microcontroller 13 which receives information fromstray field sensing antennae 21 via stray field sensing amplifiers 88and stray field sensing detectors 89.

The advantages of this alternative form of interrogator are that thereply frequency can be uncoupled from the interrogation frequency as isprovided for example by the preferred embodiment of label illustrated inFIGS. 5 and 6, and thus alternative means of elimination of interrogatornoise from the receiver channel, which means are less sensitive to fineadjustment, become available.

The construction of a preferred variety of electronic label is shown inFIGS. 13A-13D. FIGS. 13A, 13B & 13C show plan, side elevation and endelevation views of the label. FIG. 13D shows a sectional view on AA ofFIG. 13A. The label consists of a plastic label body 23 to which isattached a stamped metal foil antenna 24 connected to the integratedmicrocircuit 26. At each side of the label body spring strips 93 punchedfrom the plastic body, still attached at one end, and curved as shown,protrude. These strips have the function of preventing the label in itsnatural state from lying flat against planar objects, and in particularmetallic suitcases of cargo containers.

The advantage of this form of label is that, when the label is lyingnaturally against the object, good coupling to the interrogator fieldswhich surround metal clad objects may be obtained.

The circuit diagram and frequency response when placed between fifty ohmsource and load impedances of a receiver filter suitable for use in theinterrogator defined in FIG. 11 and operating at an interrogationfrequency of 27 MHz are shown in FIGS. 14A and 14B respectively.

The circuit shown in FIG. 14A consists of a piezoelectric crystal 94with its series mode resonance set to the interrogation frequency f_(i),in this case 27 MHz, with the combination of the crystal selfcapacitance (not explicitly shown in the circuit) and the addedcapacitance 95 being tuned by inductor 96 to a frequency an amount f_(c)above the series mode resonance, where f_(c) is the reply frequencysub-carrier.

The series capacitors 97 accomplish impedance transformation of theresonant circuit formed by the inductor 96, crystal capacitance andadded capacitance 95 so that, in conjunction with the source and loadimpedances, between which the circuit is placed, the 3 dB bandwidth ofthe passband provided for the reply signal upper sideband is establishedto be 100 kHz, while the depth of the notch established at theinterrogation frequency f_(i) is at least 60 dB.

It will be appreciated that various alterations, modifications and/oradditions may be introduced into the constructions and arrangements ofparts previously described without departing from the spirit or ambit ofthe present invention.

We claim:
 1. An identification and telemetry system comprising:an interrogator including a transmitter for generating an interrogation electromagnetic field through which code responding labels may pass; at least one code responding label including a label receiving antenna, means for sensing the interrogation electromagnetic field and means for generating intermittently repeated label reply signals; and a receiver for detecting and decoding said label reply signals; said at least one label replying intermittently as long as said at least one label is within the interrogation electromagnetic field, and said interrogation electromagnetic field being maintained for a period of time greater than the time interval between the intermittently repeated label replies; said at least one label including means within the label for determining the interval between the intermittently repeated label reply signals for said at least one label without reference to timing signals external to the label, and said interval varying from label to label and being greater than the time required for said label reply.
 2. An identification and telemetry system as claimed in claim 1 wherein repeated replies from a plurality of labels present at one time within the interrogation field do not permanently overlap in time.
 3. An identification and telemetry system as claimed in claim 1 wherein relative positions of replies from a plurality of labels present at one time within the interrogation field vary as a result of the accumulation of different intervals between replies.
 4. An identification and telemetry system as claimed in claim 1 wherein the first reply from a label occurs as soon as that label enters a region of the interrogation electromagnetic field strong enough for operation of that label.
 5. An identification and telemetry system as claimed in claim 1 wherein said at least one label includes oscillator and logic circuits within the label for determining said interval between replies.
 6. An identification and telemetry system as claimed in claim 1 wherein said at least one label comprises an oscillator within the label for regulating said interval between replies from the label, said oscillator having a voltage dependent output frequency.
 7. An identification and telemetry system as claimed in claim 6 wherein the voltage supplied to said oscillator is dependent upon the strength of the interrogation field at the position of the label.
 8. An identification and telemetry system as claimed in claim 7 further comprising means for varying interrogation power to allow variation of the interval between replies from a label.
 9. An identification and telemetry system as claimed in claim 1 wherein said interval between replies from a label is determined by a unique label serial number.
 10. An identification and telemetry system as claimed in claim 1 wherein said interval between replies from a label varies over time in accord with a pseudo-random sequence.
 11. An identification and telemetry system as claimed in claim 10 wherein said pseudo random sequence is determined by information present in the label.
 12. An identification and telemetry system as claimed in claim 11 wherein information to be provided as the reply code is used also to determine said pseudo random sequence.
 13. An identification and telemetry system as claimed in claim 10 wherein said pseudo random sequence is determined by information other than information in the label reply.
 14. An identification and telemetry system as claimed in claim 1 wherein said at least one label includes circuits within the label that present an impedance to the label receiving antenna which varies with the signal level received by that antenna so as to allow the label to operate correctly over a relatively large range of interrogation signal strength. 